Precise monitoring and control of the valves is necessary to allow an optimal engine operation. Specifically, this requires precise knowledge of the timing of the gas-exchange valves, i.e., the opening and closing instants of the intake or exhaust valves, with respect to the crankshaft position. There is increasing use of systems that allow a variation of the opening times with respect to the crank angle or, additionally, a variation of the opening cross section with respect to the crank angle. In the process, the direct mechanical coupling from cam to valves is lost, and different types of actuators decouple the camshaft from the valve motion in order to achieve the mentioned variation. Typical examples in this context are the variable camshaft adjustment or the fully variable valve adjustment.
Because the movement of the gas exchange valves is decoupled from the crankshaft angle, methods are required that allow monitoring of the valve timing as a minimum. Methods are known from the related art which determine the path or angle of the gas-exchange valves via direct transducers. Other methods are based on determining the valve opening by analyzing air quantity and rotational speed or the pressure in the combustion chamber.
The direct transducers are expensive and prone to malfunction due to their thermal and mechanical loading. Methods that require additional variables must be carefully adapted to each operating point of the internal combustion engine so as to avoid faulty diagnoses. In particular the indirect character of the method requires careful interpretation of the measured values.
German Patent Application No. DE 197 41 820 describes a method in which a pressure characteristic in the combustion chamber is analyzed. Using a gradient method, characteristic curve points that point to specific valve positions are determined. An opening or closing of a gas exchange valve manifests itself in a rapid change of the pressure gradient and is detected by a change in the operational sign of the second derivative of the pressure according to the volume.
German Patent Application No. DE 103 06 903 describes a method in which a theoretically calculated pressure characteristic in the combustion chamber is compared to an actually measured pressure characteristic. The opening time of the exhaust valve is inferred from the deviations of the two pressure characteristics. The theoretical curve is calculated assuming a polytropic expansion, by determining the polytropic exponent on the basis of two support points of the real measuring curve and the known combustion chamber geometry.
The methods that use the cylinder pressure are characterized by their direct evaluation of the combustion in the cylinder. However, the unavoidable signal noise is increased by the formation of the gradient and the higher derivative and thereby makes it more difficult to determine threshold values. On the whole, these are only small changes in the signal pattern, entailing all the problems of the numerical analysis. Furthermore, it is disadvantageous that once again a careful adaptation to all operating points of the engine is required.